In EDM's, including both wire-cut EDM's and die-sinking EDM's, various problems may arise due to the electrolytic phenomenon associated with the dielectric fluid used.
For example, when applying voltage pulses of a so-called regular polarity across a machining gap of a die-sinking EDM, i.e., wherein the machining electrode is negative and a workpiece is positive, for machining workpieces made of hard alloys, such as tungsten carbide, deionization of the dielectric in the machining gap may become difficult as machining proceeds. Particularly in portion(s) of the machining gap, such as near the center of the bottom of the machining electrode, where it is difficult to carry away the machined particles, the resistivity of the dielectric fluid may locally decrease.
In addition, when using water-based dielectric fluids which include a high polymer in a die-sinking EDM, there is a tendency to produce tar in the machining gap. Since the tar has a greater electrical conductivity than the dielectric, the resistivity of the gap may locally decrease. When electrolytic or arc currents flow through such low-resistivity portions of the gap, an oxide adherent may be quickly produced. The insulating property of this oxide adherent causes the machining rate to dramatically decrease and adversely affects the quality of the machined surface.
When using a kerosene-based dielectric fluid in a die-sinking EDM, there also is the tendency to produce tar in the machining gap, which may result in arcing. Such arcing may cause cracks on the surface of the workpiece, which will tend to shorten the useful life of die or mold being produced.
When machining a workpiece using a wire-cut EDM, in which water or a water-based dielectric fluid is used, when repeatedly applying voltage pulses across a regularly poled machining gap, i.e., one in which the machining electrode is negative and the workpiece is positive, the dielectric fluid may become electrolyzed, resulting in an electrolytic current flowing across the low-resistivity portion(s) of the gap; which in turn results in the formation of an affected layer on the workpiece surface. Particularly when machining a workpiece made of a hard alloy, such as tungsten carbide, substances susceptible to electrolysis, such as cobalt, may dissolve out of the workpiece resulting in the formation of an affected layer on the workpiece surface, which in turn may damage the workpiece and shorten the life of any die or mold being produced.
In Japanese Patent Publication No. 63-17569, a power supply for a wire-cut EDM is disclosed which has a main power source for generating a discharge across the machining gap with the machining electrode being negatively poled and the workpiece being positively poled, i.e., of regular polarity, and an auxiliary power source for generating a discharge across the gap with the machining electrode being positively poled and the workpiece being negatively poled, i.e., of reverse polarity. In this device, a voltage is first applied in reverse polarity from the auxiliary power source to initialize a discharge. After detecting the occurrence of a discharge, the main power source maintains the discharge in regular polarity while interrupting the power from the auxiliary power source. As a result, the undesirable effects of electrolytic activity are decreased, although at the cost of a reduced machining rate, since the current flow across the gap reverses direction.